Remove unused #include's from backend .c files

as determined by include-what-you-use (IWYU)

While IWYU also suggests to *add* a bunch of #include's (which is its
main purpose), this patch does not do that.  In some cases, a more
specific #include replaces another less specific one.

Some manual adjustments of the automatic result:

- IWYU currently doesn't know about includes that provide global
  variable declarations (like -Wmissing-variable-declarations), so
  those includes are being kept manually.

- All includes for port(ability) headers are being kept for now, to
  play it safe.

- No changes of catalog/pg_foo.h to catalog/pg_foo_d.h, to keep the
  patch from exploding in size.

Note that this patch touches just *.c files, so nothing declared in
header files changes in hidden ways.

As a small example, in src/backend/access/transam/rmgr.c, some IWYU
pragma annotations are added to handle a special case there.

Discussion: https://www.postgresql.org/message-id/flat/af837490-6b2f-46df-ba05-37ea6a6653fc%40eisentraut.org

src/backend/commands/trigger.c

@@ -22,7 +22,6 @@
 #include "access/xact.h"
 #include "catalog/catalog.h"
 #include "catalog/dependency.h"
-#include "catalog/index.h"
 #include "catalog/indexing.h"
 #include "catalog/objectaccess.h"
 #include "catalog/partition.h"
@@ -32,10 +31,8 @@
 #include "catalog/pg_trigger.h"
 #include "catalog/pg_type.h"
 #include "commands/dbcommands.h"
-#include "commands/defrem.h"
 #include "commands/trigger.h"
 #include "executor/executor.h"
-#include "executor/execPartition.h"
 #include "miscadmin.h"
 #include "nodes/bitmapset.h"
 #include "nodes/makefuncs.h"
@@ -44,16 +41,12 @@
 #include "parser/parse_collate.h"
 #include "parser/parse_func.h"
 #include "parser/parse_relation.h"
-#include "parser/parsetree.h"
 #include "partitioning/partdesc.h"
 #include "pgstat.h"
 #include "rewrite/rewriteManip.h"
-#include "storage/bufmgr.h"
 #include "storage/lmgr.h"
-#include "tcop/utility.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
-#include "utils/bytea.h"
 #include "utils/fmgroids.h"
 #include "utils/guc_hooks.h"
 #include "utils/inval.h"